It is expected that there will be more than 500,000 balloon angioplasty procedures performed worldwide this year. For a typical angioplasty procedure, the treating physician will identify an obstructed vessel, such as a coronary artery blocked by plaque build-up, and then inflate a small balloon within the vessel at the site of the obstruction. The inflating balloon dilates the obstructed vessel and restarts or increases blood flow through that vessel.
Balloon angioplasty has been remarkably successful at opening blocked vessels and restoring normal levels of blood flow. However, despite the initial success of the angioplasty treatment, long term efficacy is hampered by a tendency of treated vessels to re-close shortly after the initial procedure. It is understood that in thirty to fifty percent of cases, successfully treated patients will suffer within three to six months a reclosure of the dilated vessel at the site of the initial obstruction. Lincoff et al., Local Drug Delivery for the Prevention of Restenosis, Circulation, vol. 90, No. 4, (October 1994). The cause of restenosis is complex and still under study, however, it appears that restenosis results, at least in some cases, from thrombus formation. Whatever the reason restenosis is a significant factor in patient recovery and a common cause of repeat balloon angioplasty or surgical intervention.
One approach to dealing with the problem of restenosis is to maintain a passage through the artery with an endovascular stent. The stent is a generally tubular device which is placed inside the blood vessel after balloon angioplasty has been completed. The stent has sufficient strength and resiliency to resist restenosis and to maintain a passage through the vessel.
U.S. Pat. No. 4,733,665, issued Mar. 29, 1988 to Palmaz, discloses a vascular stent comprising an expandable tube. The stent is positioned over an inflatable balloon secured to a catheter and is advanced to the stenosed region. The balloon is inflated, thereby expanding the stent into contact with the vessel wall. The elastic limit of the wire mesh is exceeded when the balloon is expanded, so that the stent retains its expanded configuration. U.S. Pat. No. 4,503,569, issued Mar. 12, 1985 to Dotter, discloses a shape memory alloy stent that is advanced to a stenosed region on a catheter. The stent has the form of a coil spring. After positioning, the stent is heated with a hot fluid causing the shape memory alloy to expand into contact with the blood vessel.
Although these known stent designs can perform well, there are cases in which the stent falls to prevent restenosis. In these cases the problem of vessel obstruction is compounded by the existence of a stent that is now fixed within the vessel. Additionally, the success of a stent to prevent restenosis depends in part on the ability of the stent to span across the full length of the lesion being treated. To the extent that the stent falls to span the site of the obstruction, restenosis can occur, resulting in a reclosure of the vessel, and the need for further surgical intervention. Unfortunately, with some stent designs the radial expansion achieved by inflating the balloon leads to coincident foreshortening of the stent, often resulting in the failure of the stent to span the lesion being treated.
Accordingly, it is a general object of the invention to provide improved endovascular stents.
It is a further object of the invention to provide an endovascular stent that reduces the likelihood of failure due to tissue growth through the sides of the stent.
It is yet another object of the invention to provide an endovascular stent which is delivered to a selected site in a blood vessel and which is released from the inflatable balloon and attached to the blood vessel by the application of energy through the wall of the balloon.